Ink jet printing has become recognized as a prominent contender in the digitally controlled, electronic printing arena because, e.g., of its non-impact, low-noise characteristics, its use of plain paper and its avoidance of toner transfer and fixing. Ink jet printing mechanisms can be categorized by technology as either drop on demand ink jet (DOD) or continuous ink jet (CIJ).
The first technology, “drop-on-demand” ink jet printing, provides ink drops that impact upon a recording surface using a pressurization actuator, for example, a thermal, piezoelectric, or electrostatic actuator. One commonly practiced drop-on-demand technology uses thermal actuation to eject ink drops from a nozzle. A heater, located at or near the nozzle, heats the ink sufficiently to boil, forming a vapor bubble that creates enough internal pressure to eject an ink drop. This form of inkjet is commonly termed “thermal ink jet (TH).”
The second technology commonly referred to as “continuous” ink jet (CIJ) printing, uses a pressurized ink source to produce a continuous liquid jet stream of ink by forcing ink, under pressure, through a nozzle. The stream of ink is perturbed in a manner such that the liquid jet breaks up into drops of ink in a predictable manner. Printing occurs through the selective deflecting of some of the drops and the catching of ink drops that are not intended to strike the print media. Various approaches for selectively deflecting drops have been developed including electrostatic deflection, air deflection, and thermal deflection mechanisms.
In a first electrostatic deflection based CIJ approach, the liquid jet stream is perturbed in some fashion causing it to break up into uniformly sized drops at a nominally constant distance, the break off length, from the nozzle. A charging electrode structure is positioned at the nominally constant break off point so as to induce a data-dependent amount of electrical charge on the drop at the moment of break off. The charged drops are then directed through a fixed electrostatic field region causing each droplet to deflect proportionately to its charge. The charge levels established at the break off point thereby cause drops to travel to a specific location on a recording medium or to a gutter for collection and recirculation. This approach is disclosed by R. Sweet in U.S. Pat. No. 3,596,275, issued Jul. 27, 1971, Sweet '275 hereinafter. The CIJ apparatus disclosed by Sweet '275 consisted of a single jet, i.e. a single drop generation liquid chamber and a single nozzle structure. A disclosure of a multi jet CIJ printhead version utilizing this approach has also been made by Sweet et al. in U.S. Pat. No. 3,373,437 issued Mar. 12, 1968, Sweet '437 hereinafter. Sweet '437 discloses a CIJ printhead having a common drop generator chamber that communicates with a row (an array) of drop emitting nozzles each with its own charging electrode. This approach requires that each nozzle have its own charging electrode, with each of the individual electrodes being supplied with an electric waveform that depends on the image data to be printed. This requirement for individually addressable charge electrodes places limits on the fundamental nozzle spacing and therefore on the resolution of the printing system.
In conventional CIJ printers, there is variation in the charge on the print drops caused by image data-dependent electrostatic fields from neighboring charged drops in the vicinity of jet break off and electrostatic fields from adjacent electrodes associated with neighboring jets. These input image data dependent variations are referred as electrostatic cross talk. Katerberg disclosed a method to reduce the cross-talk interactions from neighboring charged drops by providing guard gutter drops between adjacent print drops from the same jet in U.S. Pat. No. 4,613,871. However, electrostatic cross talk from neighboring electrodes limits the minimum spacing between adjacent electrodes and therefore resolution of the printed image. Thus, the requirement for individually addressable charge electrodes in traditional electrostatic CIJ printers places limits on the fundamental nozzle spacing and therefore on the resolution of the printing system. A number of alternative methods have been disclosed to overcome the limitation on nozzle spacing by use of an array of individually addressable nozzles in a nozzle array and one or more common charge electrodes at constant potentials. This is accomplished by controlling the jet break off length in methods described by Vago et al. in U.S. Pat. No. 6,273,559 and by B. Barbet and P. Henon in U.S. Pat. No. 7,192,121. T. Yamada disclosed a method of printing using a charge electrode at constant potential based on drop volume in U.S. Pat. No. 4,068,241. B. Barbet in U.S. Pat. No. 7,712,879 disclosed an electrostatic charging and deflection mechanism based on break off length and drop size using common charge electrodes at constant potentials.
One well-known problem with any type inkjet printer, whether drop-on-demand or continuous ink jet, relates to the accuracy of dot positioning. As is well-known in the art of inkjet printing, one or more drops are generally desired to be placed within pixel areas (pixels) on the receiver, the pixel areas corresponding, for example, to pixels of information comprising digital images. Generally, these pixel areas comprise either a real or a hypothetical array of squares or rectangles on the receiver, and print drops are intended to be placed in desired locations within each pixel, for example in the center of each pixel area, for simple printing schemes, or, alternatively, in multiple precise locations within each pixel areas to achieve half-toning. If the placement of the drop is incorrect and/or their placement cannot be controlled to achieve the desired placement within each pixel area, image artifacts may occur, particularly if similar types of deviations from desired locations are repeated on adjacent pixel areas.
High speed and high quality inkjet printing requires that closely spaced drops of relatively small volumes are accurately directed to the receiving medium. Since ink drops are usually charged there are drop to drop interactions between adjacent drops from adjacent nozzles in a CIJ printer. These interactions can adversely affect drop placement and print quality. In electrostatic based CIJ printer systems using high density nozzle arrays, a significant source of drop placement error on a receiver is due to electrostatic interactions between adjacent charged print drops.
As the pattern of drops traverse from the printhead to the receiving medium (throw distance), through an electrostatic deflection zone, the relative spacing between the drops progressively changes depending on the print drop pattern. When closely spaced print drops from adjacent nozzles are similarly charged while traveling in air, electrostatic interactions will cause the spacing of these adjacent neighboring print drops to increase as the print drops travel toward the receiving medium. This results in printing errors which are observed as a spreading of the intended printed liquid pattern in an outward direction and are termed “splay” errors or cross-track drop placement errors herein. Since splay errors increase with increasing throw distance it is required that the throw distance be as short as possible which adversely affects print margin defined as the separation between print drops and gutter drops.
In inkjet printing, it is sometimes desirable to use of a halftone technique to improve the capability of producing various levels of gradation for mid tone shades. Halftone is the reprographic technique that simulates continuous tone imagery through the use of dots, varying either in size, in shape or in spacing. As an example, black and white continuous tone photographs contain millions of shades of gray. When printed, these shades of gray are converted to a pattern of black dots that simulates the continuous tones of the original image. Lighter shades of gray consist of fewer or smaller black dots spaced far apart. Darker shades of gray contain more or larger black dots with closer spacing. U.S. Pat. No. 7,637,585 by M. Serra et al. describes a halftone printing drop on demand inkjet printer that forms different sized dots on the media by depositing different patterns of adjacent drops that coalesce into the different sized dots.
In CIJ printers it has been difficult to print simultaneously with different sized drops in order to produce a multi-tone image. As such, there is an ongoing need to provide a high print resolution continuous inkjet printing system that can produce different sized drops on a recording medium using a single nozzle array of all the same size nozzle orifice. There is also a need to provide such a printing system with an electrostatic deflection mechanism to deflect selected print drops using an individually addressable nozzle array and a common charge electrode in order to provide a simplified design, improved print image quality and an improved print margin.